948 FLIGHT, 16 December I960
ANALYSING THE 727
Some Comments on Boeing's Design ... by the Technical Editor
350i
ON page 938 it is recorded that two of the largest Americanoperators have each bought 40 of Boeing's triple-jetModel 727 airliner. Many areas of its design remain
protected by Boeing, and the same is true of the powerplant;but enough is known for a technical commentary from the British
viewpoint to be attempted. Throughout the design of the 727one can see the results of the policy of reaping the maximum
reward from the vast service experience gained with the 707.
Since it is a rather larger aeroplane than the Trident 1 the 727needs engines more powerful than the Spey, and although Boeing
recently told us that the Rolls-Royce engine spectrum includeda unit—which would have been marketed, and possibly manufac-
tured, by Allison—perfectly matched to the 727, Eastern andUnited, the two operators to whom the company looked for the
bedrock on which to base the programme, both wanted Pratt& Whitney. The engine chosen is the JT8D-1, a turbofan
derivative of the J52 turbojet. The latter was planned in 1955for an advanced version of the A4D US Navy attack aircraft;
this programme was cancelled and the engine was reinstatedin revised form to drive the Hound Dog air-launched strategic
missile. It is doubtful if this application is of much value to theJT8D, but a special version of the J52, with tilting tailpipes, has
been developed to power the Intruder US Navy attack bomber.It is surprising that Boeing should describe the J52 as being
"in wide military service," since it seems to have powered onlya single aeroplane (the first Intruder).
But it is increasingly obvious that military night-time is of littleimportance in the development of an airline engine. Modern
techniques in the Willgoos high-altitude laboratory should be ableto simulate airline conditions with complete fidelity, and modern
testing techniques are such that, in Britain at least, engineersfeel they can develop a reliable engine without flying it at all.
Although the JT8D will look quite unlike the Spey, theengineers at Hartford have paid their counterparts at Derby
the complimenr of making it thermodynamically virtually identical.Three of the most important factors which determine the by-pass
ratio of engines for this market are sketched on this page. Theoriginal airline Conway has the low ratio of 0.3, while the JT3D
has a ratio of 1.5. Although it would be futile to deny that thetwo companies have spent a lot of time looking over each other's
shoulders, they have made their own calculations and indepen-dently concluded that the best value is near unity (indicated
by a tinted strip in the illustration).
Pratt & Whitney's first attempt to do a turbofan for this marketwas the JTF-10, rated at 8,2501b and intended for the defunct
DC-9. This engine had a by-pass ratio of about 1.5, and alsostemmed from the J52. In view of the fact that the straight
turbojet has military ratings of between 7,500 and 8,0001b it seemsclear that the new 727 engine is altogether larger, and it may
be that no J52 parts are incorporated at all. It is worth notingthat in the 727 die fan air is ducted aft, as in a Rolls-Royce
engine, to pass through a single reverser and mix with the hotjet in the nozzle. Boeing have done well to fit a reverser on the
middle engine.
Switching to aerodynamics, it is probably fair to commentthat, while the game of finding the right wing to meet the
cruising case is a straightforward slog with tunnel models andcomputers, the number of variables which can be introduced
to meet the landing case makes the final wing design elusive,de Havilland eventually arrived at a wing with an MNO of 0.875
ROLLS-ROYCE -*-*-P&W
Curves, plotted against by-pass ratio, of
engine + fuel weight tor a 1,000 n.m.
stage (expressed as percentage of air-
craft gross weight), increase in total
engine drag (A), increase in total pod
drag (B), and reduction in s.f.c. allow-
ing for (C) accessories and air bleed
and (D) for increasing pod drag also.
As indicated, Rolls-Royce and Pratt and
Whitney are both approaching a by-pass
ratio of unity, one from below and the
other from above
In their search for greater maximum lift coefficients
de Havilland have for the Trident adopted a hinged
leading edge, double-slotted flap and upper-surface
brake (top). For the 727, Boeing have chosen slats out-
board, leading-edge flaps inboard, triple-slotted flaps
and spoilers
above 24,000ft and a best-economy point of M0.87 at 32,000ft—a remarkable speed when it is considered that "maximum
economy" means what it says. The Hatfield aerodynamicistsprobably thought they had gone far enough in adopting leading-
edge flaps and double-slotted trailing-edge flaps, but Boeing havetried to go one better. Whether or not they have succeeded
depends on such other factors as the first cost of the aircraftand the trouble it will give in service.
As we noted in our October 14 issue, Boeing have aimed ata maximum lift coefficient of 2.5 in order to meet a challenging
requirement in the matter of field-length; and it is rumouredin Britain that they have achieved 2.8. The accompanying sketches
of wing profiles indicate the degree of complexity necessary 10achieve such a figure; and, although Boeing do not mention
it, rough calculations suggest that the 727 may well have partiallyblown flaps. When we visited the company last month the
"dash-80" prototype of the 707 family was flying with blownflaps under' the wing fillets and sonic blowing around all the
"dirty" areas of the main flaps, including jets intended to cleanup the airflow over the flap rails. Yet another indication of the
importance of the field-length limitation is the fact that the 727'ssweep angle is less than that of either the 707 or the Trident.
From the early days of the Comet 1 de Havilland have beenwedded to fully powered flying controls, and in the Trident a fully
triplexed system is employed—three jacks on each surface—toprovide a two-over-one majority rule in the event of failure. Each
of three independent hydraulic supplies is served by its ownengine, and the D.H./BEA/Smiths team have by specific reliabi-
lity requirements of ARB been forced to apply extensive multi-plication—dieir solution is triplex throughout—in order to meet
automatic-landing safety levels. Such a situation has not yet beenfaced at Seattle and, as was reported in our issue of November 25
(page 840), Boeing are not trying to do anything comparablewith the 727. The flying controls of the new airliner are really
manual with hydraulic boost, so that effective manual reversionis possible. It may be recalled that on the 707 series not even
hydraulic boost is provided on ailerons and elevators, and Boeingseem to think that fully powered controls are something one
should always try to do without.
British designers, on the otfier hand, took the decision to applyfully powered controls without manual reversion in conjunction
with an all-flying tailplane. The addition of a multiplicatedautomatic landing system to the 727 appears to be extremely
difficult, involving as it does the application of genuine multiplica-tion to a fundamentally split-authority control system. The addi-
tion of a set of triplicated or triplexed autopilot servos to themanual/boosted controls would raise some interesting statistical
problems in reliability calculations.
From the scanty details available it appears that the accessorysystems of the 727 depart hardly at all from previous practice,
although doubtless one or two surprises are in store (not a wordis available on ice protection). For the Trident, DH and Rolls-
Royce are confident enough to pressurize the cabin by direct airbleed, as is at present proving quite successful with the Caravelle.
Boeing are known to have looked at such systems for many years;but they have yet to succeed in discarding the bleed-air turbo-
compressors which pump fresh air to the cabins of 707s and 720s,and the same dual-cycle arrangement will be perpetuated in at
least the first 727s. Later, if it can be demonstrated that theinside of the 727 could never become filled with vapour from hot
turbine oil, Boeing may adopt a direct bleed system.
0-5 10
BY-PASS RATIO
1-5